(Al.sub.x Ga.sub.1-x).sub.0.51 In.sub.0.49 P quaternary compound is a direct bandgap semiconductor material which is lattice matched to a GaAs substrate. The compound has high radiative recombination efficiency in the wavelength range from 560 nm to 650 nm and is a good candidate for fabricating high brightness light emitting diodes for emitting light of red to yellow green color.
FIG. 1 illustrates the cross-sectional view of a conventional AlGaInP light emitting diode comprising an n-type GaAs substrate 100, an n-type AlGaInP lower cladding layer 101, an undoped AlGaInP active layer 102, and a p-type AlGaInP upper cladding layer 103. Because it is difficult to heavily dope the AlGaInP material in p-type, the upper cladding layer 103 has high resistivity that restricts the current flow just underneath the top metal electrical contact of the light emitting diode. Therefore, the emitted light generated from the active region underneath the top metal electrical contact is mostly absorbed by the opaque top electrical contact. The luminous intensity of this kind of light emitting diode is not very high.
In order to promote current spreading, U.S. Pat. No. 5,008,718 adopts a high energy band gap GaP transparent window layer which has low resistivity to help distributing the current uniformly across the active layer. Because the window layer used is a heavily doped p-type GaP material which has a very low mobility, its resistivity is only about one order of magnitude lower than that of the upper cladding layer. Therefore, a window layer with substantial thickness is necessary for improving the current spreading.
A thick transparent window layer not only takes a long time to grow but also costs more. To reduce the window layer thickness, U.S. Pat. No. 5,359,209 discloses a double layer window structure including a low energy band gap heavily doped p-type GaAs layer and a high energy band gap GaP layer. The heavily doped p-type GaAs layer has a very low resistivity which is about two orders of magnitude lower than that of the upper cladding layer.
Because of the low resistivity in the heavily doped p-type GaAs layer, the overall resistivity of the double layer window structure becomes much lower. The total thickness of the window structure can be significantly reduced. However, the inclusion of this thin heavily doped p-type GaAs layer reduces a few percent luminous intensity of the light emitted from the diode because the GaAs layer is opaque.